Abstract

We apply an information-theoretic measure for phase synchrony to local field potentials recorded with a multi-electrode array implanted in area V4 of the monkey visual cortex during a reinforcement pairing experiment. We show for the first time that (1) the phase synchrony is significantly higher for the rewarded stimulus than the unrewarded one, after training the monkey; (2) just after the stimuli reversal, the difference in phase synchronization is due to the stimuli, not the reward; (3) the difference between reward and no reward is most clear in two disconnected time intervals between stimuli onset and the expected delivery of the reward; and (4) synchronous activity appears in waves running over the array, and their timing correlates well with the time intervals where the difference between reward and no reward is most prominent.

Received 15 February 2008Accepted 02 June 2008Published online 22 September 2008

Lead Paragraph: An electrode array was implanted in a visual region of the monkey brain, and from which local field potentials (LFPs) were recorded. Local field potentials are extracellular current flows. We compute the phase of the LFPs using the Hilbert transform. We determine the normalized mutual information between pairs of LFP phases, and consider them to be in synchrony when the mutual information is high. We analyze the evolution of the phase synchrony in a stimulus-reward pairing experiment, and show that synchronous activity appears as waves propagating over the array.

Acknowledgments:

The authors are deeply indebted to Professor R. Vogels, of the same lab, for sharing his experimental data. N.V.M. is supported by the European Commission (IST-2004-027017). M.M.V.H. is supported by research grants received from the Excellence Financing (EF 2005) and CREA Financing (CREA/07/027) programs of the K.U.Leuven, the Belgian Fund for Scientific Research – Flanders (G.0248.03 and G.0234.04), the Interuniversity Attraction Poles Programme – Belgian Science Policy (IUAP P5/04), the Flemish Regional Ministry of Education (Belgium) (GOA 2000/11), and the European Commission (NEST-2003-012963, STREP-2002-016276, IST-2004-027017, and IST-2007-217077).